A power unit as may be comprised of an engine and a variable speed transmission is resiliently mounted on a suspension member of a car body by using a main mounting apparatus and a buffer rod. The phase of oscillation of the buffer rod is reversed across a resonant frequency, and this fact is utilized to cause the buffer rod to serve as a dynamic damper through a suitable choice of the resonant frequency thereof, in an attempt to dampen the oscillations of a vehicle during its running and to prevent the occurrence of confined sounds of low frequencies.
However, because the buffer rod has a constant effective mass, an attempt to reduce the confined sounds results in excessively increasing the magnitude of the spring constant of the mounting apparatus in a range of frequencies which are just below the resonant frequency, thus rather degrading the damping characteristic of the power unit mounting apparatus.
The buffer rod consisting of a pair of elastic members and a mass member interposed therebetween exhibits good oscillation suppressing effects at frequencies above the resonant frequency thereof. However the buffer rod does not exhibit a good oscillation transmission rate for oscillations of frequencies which are just less than the resonant frequency of the elastic members.
Therefore, it is a first object of the invention to provide an apparatus for mounting a power unit which is capable of preventing oscillations of a car body when it is running at a medium speed by using the mass member having a first weight and also capable of preventing the occurrence of confined sounds when the vehicle is running at a high speed by using the mass member having a second weight, thus improving the anti-oscillation performance over the entire range of frequencies which practically occur as a result of running of the vehicle.
It is a second object of the invention to provide an apparatus for mounting a power unit which accommodates for a degradation in the oscillation transmission rate in a range of frequency below the resonant frequiency of the elastic members which have a mass member interposed therebetween, while taking advantage of a reduction in the transmission of oscillations in a range of frequencies above the resonant frequency.